Tire Pressure Monitoring Systems (TPMS) play an important role in vehicle safety and emissions reduction. Several countries and governing bodies have enacted mandatory regulations that require vehicles to have TPMS; for example U.S., European Union and Korea. A majority of this market is served by direct tire pressure monitoring systems, in which each tire contains a TPMS sensor module. Due to this high market penetration, the cost and the size of these sensor modules is of high importance. Current sensor modules consist of a tire pressure sensor (TPS) integrated circuit, a battery, antennas for communication, and a very small number of discrete passive electrical components.
Most sensor modules use a motion detection sensor to conserve the life of the sensor module's battery by entering power-down mode while the vehicle is parked. In this way, the service life of the sensor and its non-replaceable battery can be maximized. Today, most common way to measure the motion of wheel it is to use a radial direction acceleration sensor (e.g., a z-axis acceleration sensor, x-axis acceleration) or shock sensors. Thus, the typical motion detection sensor responds to g-force and is either an accelerometer or a shock sensor.
In order to detect if the vehicle is driving or parked the acceleration sensors is sampled according to a sampling period. Normally a z-axis acceleration sensor is used to measure an absolute value of the centrifugal force which represents the vehicle speed. The minimum detectable speed is limited by the acceleration offset error which can be set during calibration of the sensor in the semi-conductor production. State of the art calibration techniques yield an offset error of +/−3.5 g, which limits the motion detection threshold to approximately 20 km/h. That is, motion detection cannot be detected below 20 km/h.
Another disadvantage of acceleration sensors is that they require a calibration process to measure absolute acceleration at radial direction (e.g., offset calibration, temperature calibration, sensitivity calibration, etc.). The calibration process is performed in the semi-conductor production line. To avoid additional life time drifts and to keep the same high accuracy for the offset error, the application may also need to have auto-calibration process to remain the accuracy over its lifetime. The calibration process requires more cost and time at the sensor manufacture and auto-calibration in application requires more energy. The offset error dominates the minimum vehicle speed for motion detection, which is currently limited to 20 km/h with this solution.
TPMS sensors that employ microelectromechanical systems (MEMS) acceleration sensors must be handled carefully prior to installation into a wheel, to prevent breakage of the accelerometer due to mechanical resonance. Thus, the risk of breakage is high.
In addition, TPMS sensors that employ off-chip shock sensors contain circuitry that conditions the signal from the shock sensor, so that motion can be detected. TPMS sensor software for sensors with shock-sensing technology normally requires several shocks to be observed before the sensor is considered to be in motion. Also, the shock sensors are very-high impedance devices, and therefore the signal conditioning circuity must also be of high input impedance. Unfortunately, this normally results in a circuit that is inherently susceptible to electromagnetic interference (EMI). The result is a sensor that may be operating and expending energy when the vehicle is not moving. Furthermore, the external mounting of the shock sensors requires additional printed circuit board (PCB) space and they are also quite costly compared to an integrated solution (e.g., acceleration sensors). The additional calibration on PCB-level requires more cost and time as special equipment is required in the production line of a Tier 1.
Therefore, an improved motion detection sensor may be desirable.